The present invention relates in general to an internal combustion engine which is constructed to produce a minimum amount of harmful compounds, and particularly to a cylinder head, for an engine, having port liners within the exhaust port passages. More specifically, the present invention is concerned with an improved cylinder head and a method of forming exhaust port passages in a body of the cylinder head, the exhaust port passages receiving therein port liners for obtaining preferable thermal insulation between the exhaust gases passing therethrough and the body of the cylinder head.
For the purpose of reducing harmful compounds, such as hydrocarbons (HC) and carbon monoxide (CO), contained in the exhaust gases emitted from the internal combustion engine, after combustion devices such as a thermal reactor and a catalytic converter are employed in the exhaust system of the engine. In the case of a thermal reactor, the oxidizing efficiency is critically dependent upon the temperature of the exhaust gases being admitted into the device, and thus, it is necessary to minimize the heat loss in the exhaust system to keep the exhaust gases at the highest possible temperature.
Conventional internal combustion engine systems have thus, port liners within the exhaust port passages formed in a monocast cylinder head and, more sophisticatedly, have an insulating space or clearance formed around the port liner for thereby obtaining more effective thermal insulation between the exhaust gases passing through the exhaust port passages and the body of the cylinder head.
In connection with the method of disposing such port liners in the exhaust port passages of the cylinder head and of forming such insulating space or clearance around the port liners in the cylinder head, it has been proposed that, the cylinder head be formed by casting, and each port liner member filled with so called moulding sand and covered by a shaped casting core or cores made of moulding sand is placed in its proper place in the casting box for the cylinder head. After the casting, the moulding sand received in and on the port liner is removed through the outlet of the port liner and holes, which holes are arranged in the vicinity of the outlet of the exhaust port passage in the cast cylinder head. With this sand removing operation, the port liner is left in the exhaust port passage and simultaneously, the heat insulating space is formed.
In this conventional method, however, there is a problem that if the port liner has a complicated curvature due to the complicated construction of the exhaust port passage, the curing operation of the moulding sand received in the port liner must be done after the moulding sand is filled into the port liner, in other words, the heating operation for the moulding sand to cure the same is inevitably carried out when the moulding sand is packed in the port liner. Thus, it takes a long time to completely or preferably cure the moulding sand because of the heat insulating action of the port liner. In reality, it has been often observed that the casting core sand nearest the port liner is subjected to over-cure when the moulding sand near the center of the port liner is preferably cured. Furthermore, if the port liner is preheated to about 300.degree. C for the purpose of shortening the curing time of the moulding sand therein, some specially designed machines are required to protect the worker from dangerous heat radiating from the heated port liner.